Water Pyrometer. 587 Example 58. — Find the specific heat of copper from the following data : — Half a pound of copper is heated to 212°, and being plunged into a pint (20 02.) of water at 60° contained in a wrought iron vessel weighing- 4 oz., raises the temperature of the latter to 65 f°, («/,*! +. w±s2) (T - O _ (1*25 + "25 x -113) (65! - 60) J= """^(/°-~T0) - ..... •5(2i2~-~65|) ...... = °938 Pyrometers, for measuring very high temperatures. — Wedge- wood's and DanielPs, based on expansion of solids, are now obsolete. Siemens' electric pyrometer measures the resistance of a circuit, which varies directly as the temperature of the wire. Wilson's and Siemens' water pyrometers depend on the method of mixtures. A cylindrical vessel of sheet copper, clothed with felt to retain heat, is provided with a cover and thermometer (see B, Fig. 598). A small solid cylinder of copper, of known weight, being placed in the furnace whose temperature is required, is shortly removed, and plunged into the water of the pyrometer, ^vhen the latter is closed. The final temperature of the pyrometer -water being observed, that of the furnace can be deduced. (See Appendix //.,/, 876.) Example 59. — Find a flue temperature by water pyrometer from the following data: — Quantity of water = I pint, its first tem- perature 65° ; weight of copper cylinder — 4 J oz, ; final temperature of •water = 72° : water equivalent of vessel = '38 (Ib. F°) +.:3.8) . 4- wsl0 528 __ 2 5 ± '38) 7 H- C'26s x -095 x 72) ws "265 x '095 Expansion of Gases.—Two laws govern the varying volume of a gas, according to whether temperature or pressure be kept constant, The first law of gas expansion, discovered by Boyle ia 1662, and verified by Marriotte in 1676, states that the volume of a given portion vj gas varies inversely as its pressuref if the temperature be constant. Shewa by symbols: V oc = and PV » a constant. The relation of P and V is given by diagram in Fig, 599, the ordinates PPX of the curve representing pressures, and the 1 ' RR